Add --setup (-s) option, like --prepare but runs once per batch

Add a --setup option for the use-case of wanting to do one-off setup
before a set of benchmarks, not the once-per-test setup that --prepare
does. This is useful for the cases noted in the updated documentation.

Per the feedback on https://github.com/sharkdp/hyperfine/pull/448 this
new "--setup" option will steal the "-s" short option from
"--style" (initially this was called "--build" and used the
non-conflicting "-b").

Potential future work:

I'd prefer if this and --cleanup took N number of commands, so you
could do e.g.:

    hyperfine -L rev master,next \
        -b 'git worktree add /tmp/test-{r}'  \
        -b 'make -C /tmp/test-{r} all'  \
	'make -C /tmp/test-{r} test' \
	-c 'git worktree remove /tmp/test-{r}'

I.e. a shortcut around not providing these with &&, which makes things
a bit more readable.

But the --cleanup option doesn't do that, so let's just go with what
it's doing for consistency, so for this you'll now need to do:

    hyperfine -L rev master,next \
        -b 'git worktree add /tmp/test-{r} &&
	    make -C /tmp/test-{r} all'  \
	'make -C /tmp/test-{r} test' \
	-c 'git worktree remove /tmp/test-{r}'

src/options.rs

@@ -114,6 +114,9 @@ pub struct HyperfineOptions {
     /// Whether or not to ignore non-zero exit codes
     pub failure_action: CmdFailureAction,
 
+    /// Command to run before each batch of timing runs
+    pub setup_command: Option<String>,
+
     /// Command to run before each timing run
     pub preparation_command: Option<Vec<String>>,
 
@@ -146,6 +149,7 @@ impl Default for HyperfineOptions {
             runs: Runs::default(),
             min_time_sec: 3.0,
             failure_action: CmdFailureAction::RaiseError,
+            setup_command: None,
             preparation_command: None,
             cleanup_command: None,
             output_style: OutputStyleOption::Full,